This invention relates to the formation of synthetic tapes or films from liquid polymeric materials. More particularly, this invention relates to a novel method and apparatus for forming such tapes or films utilizing a die slit configured to enhance productivity and/or film properties.
In the art of melt extrusion of synthetic tapes or films it has been found that productivity is limited by reason of phenomenon termed melt resonance or draw resonance. Draw resonance in connection with such extrusion operations is a type of instability condition which manifests itself as a cyclic variation in the width and the thickness of an extruded tape such that the width and the thickness of the tape vary periodically along the tape length.
When one extrudes and then draws a thermoplastic shape into a quenching medium, the extrudate necks down in the gap between the die and the solidification region. As the windup speed is increased, the cross-sectional area of the extrudate in that gap becomes smaller. With all other extrusion operation parameters fixed, there is usually a windup speed or range of speeds at which the onset of draw resonance instability will occur. At a critical windup speed, a cyclic pulsation appears in the extrudate. This pulsation becomes more pronounced at increased speeds until eventually breakage occurs.
The limiting factor or productivity presented by the need to operate below critical windup speed is readily apparent. Moreover, the problem of windup speed limitation is additionally troublesome in connection with the production of products such as low denier fibrillated yarns. It is desirable to establish higher tape draw ratios in the production of a low denier product. This in turn dictates higher windup speeds which may induce draw resonance.
It is therefore an object of the present invention to provide a novel operation for the formation of synthetic tapes where productivity is enhanced by control of draw resonance.
It is a further object of the present invention to provide such a novel tape forming operation that is particularly adaptable to the high throughput production of low denier products.
In this connection, the present invention embodies the discovery that the onset of draw resonance is related to the ratio of the extensional strain rate of the material leaving the die to shear strain rate of the material inside the die. It has been found that for a given draw down ratio and with other operating parameters constant, the onset of draw resonance instability correlates within a narrow band of values of this ratio of extension rate to shear rate. Above this band stability is present, and below the band deleterious draw resonance occurs.
As draw ratio is increased, the same relationship holds, the only difference being the increased magnitude of the instability band of values of the ratio of extension rate to shear rate.
The present invention further embodies a recognition that the ratio of extension rate to shear rate is a function of quench height, i.e., the gap between the die exit and the region of solidification. Accordingly, the extension rate to shear rate ratio can be controlled by the profile of the exit slit.
Concurrently with the development of the present invention it was determined by the applicant that one explanation for the surprising performance, from the standpoint of draw resonance, of an extrusion die having a slit with radiused ends as disclosed in commonly assigned copending application Ser. No. 633,758 [filed Nov. 20, 1975 for "High Tenacity, Low Denier Poly (ethylene terephthalate) Fibrillated Tape Yarn" by John D. Gibbon] is that the radius at each end of the die slit alters the ratio of extensional strain rate to shear strain rate. While the existence of the particular configuration of die slit set forth in that application preceded the present invention, the present invention proceeded independently, and it was not until the present invention that the foregoing explanation for improved performance of the earlier die was advanced.
In accordance with the present invention the profile of a die exit slit through which liquid polymeric material is passed is configured to establish a controlled ratio of extensional strain rate to shear strain rate entirely across the slit. Through a predetermined configuration of the depth profile of the exit slit, the extension rate to shear rate ratio can be maintained at an acceptable level across the entire slit. The present invention contemplates a similar control over the extension rate to shear rate ratio in connection with a predetermined width profile of the slit.
Additional objects of the present invention will become apparent below, such as the provision of a novel film forming operation wherein draw resonance problems are minimized while a high orientation is imposed on the material exiting from the slit. This is accomplished through control of the extensional strain rate of the material through provision of a predetermined depth profile of the slit.
It will be appreciated that the foregoing objects of the present invention dealing with novel control of draw resonance, throughput, and orientation are intended to embrace tape or film forming operations other than where a melt is extruded through a die and passed to a quenching zone. That is, the present invention contemplates other tape forming operations wherein liquid polymeric material is passed through a slit toward a solidification region, for example tape forming operations where a liquid polymeric solution is passed through a slit toward a solidification region established by the evaporation or dissolution of the solvent for the polymer.
Since it is envisioned that the speed of the variety of tape forming operations contemplated according to the present invention could be significantly increased over present commercial tape forming operations, the term "film or tape spinning" is at times used herein to denote the process and the term "spinneret" is at times used herein to denote the member having the slit with the profile established according to the present invention.